1. Field of the Invention
The present invention relates to a fiber comprising polyvinyl alcohol (PVA) and polyvinyl chloride (PVC) which is treated with calcined stannic acid of a specific crystal size to improve flame retardancy and resistivity to discoloration resulting from heat treatment of the fibers.
2. Description of the Prior Art
Stannic acid and tin dioxide are well known flame retardants for cellulosic fibers. It is also known to add stannic acid as a flame retardant to a mixed PVA halogen-containing polymer fiber as suggested in U.S. Pat. No. 3,859,390 granted Jan. 7, 1975. The aforementioned flame retardants exhibit certain disadvantages, however, such as relatively poor fire retardancy unless more than 10% of the flame retardant is added to the fiber. Horemover, these known flame-retardants tend to cause glowing or reduction in the fiber strength.
The addition of stannic acid as a flame retardant in a mixed PVA/PVC fiber satisfactorily promotes flame-retardancy but causes considerable discoloration of the fiber during heat treatment thereof. When polyvinyl alcohol-polyvinyl chloride fiber is post-treated with tin tetrachloride to incorporate stannic acid in the fiber, discoloration resulting from such post-treatment is likewise significant. When the aforesaid fiber is manufactured by a process wherein the stannic acid is dispersed in the PVA/PVC spinning solution so as to incorporate stannic acid in the fiber, discoloration similarly results upon final heat treatment.